The present invention concerns a method for generating an optimized printer calibration.
In general, printers (inkjet systems, laser systems, other digital printer systems) are linearized or calibrated by means of a density-based linearization of the primary colors. These printers operate generally with the primary colors cyan, magenta, yellow, and black (CMYK). Calibration in this context is to be understood as restoring a defined nominal state. For the example of density calibration, this is restoration of the nominal densities. For the linearization, the maximum color density for each color is determined and then, linearly descending, the corresponding density value for intermediate steps. Subsequently, the percental control values (tonal values) at which the defined color densities can be achieved are searched for. This correlation is saved for each print color in a one-dimensional table. In case of a calibration, this search is repeated again in order to reach the defined densities again; however, in this case the changed behavior of the printer is taken into account. Instead of measured densities, measured color values (e.g. CIELAB values) can also be employed for linearization and calibration. Example: For 100% cyan a maximum density of 1.5 is measured. In order to achieve half of the density, i.e., 0.75, only 43% of cyan must be printed. In the linearization or calibration table, the output value 43% is thus correlated with the input value 50%.
In this method, it is not taken into account that the behavior of combined printing of the printer may change even though the primary colors have reached their nominal state again after calibration. Despite calibration, a visible difference results therefore in combined printing. An improvement of calibration is achieved in that a three-dimensional or higher dimensional correlation table is formed that contains also nominal values for colors that result from combined printing. Upon calibration, a combination of the primary colors that reproduces the defined nominal color is then searched for. As nominal values, measured color values are employed in this case that correspond to the visual perception of the human eye (CIELAB). In this case, for a combination of the input tonal values, a nominal CIELAB value and the combination of the primary colors required for reaching the nominal value are saved in the calibration table.
Based on this, as a further important aspect, a defined and constant description of the printable color gamut of the printer is provided that can be used for further tasks, for example, for the calculation of simulation profiles. In this connection, a different color reproduction method is simulated on the printer.
In the known method, the nominal values are measured. This does not imperatively achieve that the values have the same spacing. Also, no neutral gray balance is provided. On the other hand, the measured nominal values are subject to the usual fluctuations occurring for any measurement.
The terminology that is employed in the present context is based on the following brief descriptions:
Colors, referred to in colorimetrics more precisely as color stimulus specification, are based on color stimuli that differ with respect to their spectral composition. Because of the necessity to be able to precisely define these differences, various color models have been developed. Each color can be defined by a color name (descriptive words) but also by the numerical color point. Depending on the color model the color can be described with respect to brightness, saturation, and hue but also in accordance with light/dark, red/green, and yellow/blue value (as, for example, in the frequently employed CIELAB color model) with three such parameters in a distinct way.
In the CIELAB color model, the three parameters L*, a*, and b* define a three-dimensional Cartesian space that is referred to as color space. A color point characterizes in this context a point within this CIELAB color space that is identified by its three coordinates. The spacing between two color points corresponds in approximation to the visually perceived color difference between the realization of these colors. Color spaces serve thus inter alia for visualizing differences between an ideal state, for example, the desired nominal values, and the achieved reality.
The color-generating method can realize only a portion of all conceivable color points. Some colors have a defined color point but cannot be represented with the available color means. The representable colors form within the color space a body that is also referred to as gamut. This gamut is referred to as color gamut.